If the law in your state requires barrel length to be included on the registration form, I'm sure it would not be legal to shorten it.
Rather than shortening the barrel, it would be more likely that a gunsmith would just find a shorter barrel, and replace it. Probably at a cost less than cutting, and re-crowning.

???

In terms of a state registration, how would your approach be any different from his? He would still end up with a revolver having a 3" barrel and a registration saying it has a 5" barrel. I don't think the State of Michigan would care how the result was achieved -- what matters is the result.

I think the question should be directed to the State, asking if there's a procedure to amend the data in the registration system. To me it's like an automobile registration. My car is registered as a Blue Jeep Cherokee. If my daughter decided to paint it hot pink, I would have to notify the department of motor vehicles that the Jeep is now pink.